Personally I find such traits attractive, however I usually favor true/genuine sadness and or problems... that may sound wrong to most people reading this, but let me explain, I enjoy those sorts of things in women, because I take great joy in cheering others up, motivating people, and helping others through problems.
that happens to me a lot to. my face when I'm expressionless or concentrating, or just spacing out, looks sad (or so I've been told). I don't think its affected my relationships with people, I have wonderful friends and a wonderful boyfriend who know me and know that I am generally a happy person. so it hasn't been aproblem for me and it doesn't bother them.
Why do you tend to look sad and thoughtful a lot? You shouldn't really be that morbid and morose at such a young age.
You should be optimistic, young, lively and full of spirit!
But I would tend to think that guys who see you in your 'generally sad' and 'thoughtful state' would think of you as a depressed, unapproachable, unfriendly, emotional girl. And you probably don't want to be associated with that persona, look at your username - sunshine2!
Try to appear more friendly and happy. Don't look thoughtful. Look happy and friendly, light and bubbly. Even ditzy would be better than sad.